The present invention relates to signal transmission, and more particularly, relates to direct up-conversion analog signals.
In the field of radar, radio and other signal transmission applications, it is often desirable to up-convert a baseband signal from one frequency to a higher frequency. Typically, this has been done using multiple local oscillators with associated filters, mixers, intermediate frequency amplifiers and phase-locked loop circuitry. Such circuits consume power, are inherently lossy, and can emit spurious unwanted harmonic signals. These unwanted emissions can, with appropriate equipment, be detected and hence reduce the stealth capabilities of such applications. Additionally, a number of components associated with these local oscillators cannot readily be implemented in integrated circuits, requiring off-chip circuit elements such as crystals and inductors.
When utilizing field portable power sources, it is desirable that power consumption be minimized, and power be utilized efficiently. Reducing the circuit element count in a circuit can reduce power consumption, but utilizing traditional local oscillators and mixers for up-conversion has a practical limitation in traditional local oscillator-based designs for up conversion.
What is needed is an efficient (i.e. low circuit element count and power consumption) way to provide up-conversion, while minimizing spurious emissions to improve the stealth characteristics of the application.
The present invention addresses the problems associated with using local oscillators by performing direct up-conversion using a novel combination of analog and digital circuitry to produce a sampling pulse, which is used to control a gated differential amplifier. This results in a time domain waveform that is a pulse doublet train which is amplitude modulated by the input signal to the differential amplifier. The pulse circuit is generated by frequency doubling, amplifying and limiting (to square up the resulting signal) a base signal one or more times, and using the resulting signals (times two, times four and so forth) to produce a short duration sampling signal repeating at the frequency of the base signal.
In one aspect, the invention includes a system and method which squares an input sine wave, applies it to a frequency doubler and limiting amplifier (FDLA), and the two signals are fed to a logical NOR gate to produce a narrow sampling pulse. The sampling pulse is used to control the output of a gated differential amplifier. When the sampling pulse is asserted, the output of the gated differential amplifier tracks the input (from, in one aspect, a digital to analog converter) and when the sampling pulse is not asserted, the output of the differential amplifier is pulled to zero. In another aspect, the doubled input signal is in turn applied to a FDLA to produce a times four signal which is also input to the logical NOR gate, in which aspect the sampling pulse is repeated at the frequency of the input sine wave, but has a duration of a single half cycle of eight times the frequency of the input sine wave.
In another aspect, the invention is, at least in part, implemented as a Monolithic Microwave Integrated Circuit (MMIC), and in yet another aspect uses pseudomorphic high electron mobility transistors (PHEMT). In still another aspect, the differential amplifier is controlled by gating the biasing current to the amplifier. The output of the differential amplifier, in another aspect, is converted to unbalanced microstrip using a planar balun.
An advantage of the present invention is that it may be implemented in a single monolithic integrated circuit without the need for external local oscillators and mixers. It is also an advantage in a wideband system that eliminating the need for local oscillators reduces the potential for in-band and near in-band re-radiation. Yet another advantage is a reduced number of circuit elements which reduces cost and power consumption.